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RE: ASCE 7-05 Diaphragm Design

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Rho would apply because it is applied to the base shear.  Omega would not apply to the diaphragm.  Rho is always one for categories B or C.


Having a full height frame and then some less than full height frames is not a discontinuity in my opinion, anymore than using less bays of bracing in a frame as you go up in elevation.  Each frame is still continuous from start to foundation.  A discontinuity exists when the five story brace stops at level two and needs to transfer to another line.


Yes, force transfer can occur when you introduce an additional element in the lower parts of the building, but be careful in the analysis because computers will show large force jumps that are not realistic.  But this is not a discontinuous system.


From: Gerard Madden, SE [mailto:gmse4603(--nospam--at)]
Sent: Tuesday, April 22, 2008 1:57 PM
To: seaint(--nospam--at)
Subject: ASCE 7-05 Diaphragm Design


The way I read section, it means that diaphragms shall only be design for the RHO factor when they are transfer diaphragms in Seismic Design Categories D, E, & F.

This seems logical (and OMEGA would kick in too due to the discontinuity).

This can be difficult to track if another frame line is taking load out of adjacent frame (even one that is not discontinuous) through the diaphragm (like unzipping it). This can happen say when you have say:

A 5 story braced frame from foundation to roof and then near bye a frame from foundation to 2nd floor. At the second floor diaphragm, force transfer can occur even though the 5 story frame is continuous.

Anyone beg to differ?

I guess the moral of the story is have RHO = 1.0 and get 2 frames per side...